Impact line printers comprise a continuously revolving type carrier having engraved type elements such as alphanumeric characters moving past a row of individually operable print hammers. A control system, which might include a print line buffer for storing a line of data to be printed, a band image buffer which stores an image of the arrangement of the characters on the type carrier along with timing and hammer firing circuitry, selectively operates the hammers in synchronism with the motion of the characters. Imprints of the type elements are produced on a print medium by causing the print medium and the type elements to be impacted against each other as a result of the selective operation of the print hammers while the type carrier is in motion. The print medium is incremented one or more line spaces at the completion of a print line and the process repeated for successive lines of print.
Bar code patterns can be of two types, the picket fence made up of a pattern of vertical bars and spaces, and the ladder code made up of a pattern of horizontal bars and spaces. In either case, the bar code patterns can include bars and spaces of different widths. The type carrier then includes vertical or horizontal type bars. A problem arises when such bar code is printed on a high speed impact line printer. The problem is especially difficult when the wide bar is three or four times the width of the single width bar. If the hammer energy level is high enough to form good density imprints of the wide bars, then paper cutting or embossing can occur when printing thin bars which affects the machine readability of the imprints. Alternatively if the energy level is reduced to avoid cutting or embossing the paper, then the imprint of the wide bars results in faint imprints which again affect machine readability. One solution, of course, might be to selectively alter the energy of the hammers dependent on the width of the type bars. Such an approach would be conveniently done in serial type impact printers which operate at relatively slow speeds. However, for high speed impact printers, altering hammer impact energy levels for individual hammers dependent on the width of the bar would slow the printing process and introduces considerable complexity into the controls and complicates the flight timing of the hammers which varies dependent on the hammer energy level.
Examples of multiple width printer devices are shown in U.S. Pat. Nos. 3,834,506; and 3,820,643. These patents show multiple width bar code printing using two print bars operated simultaneously to print bars side by side to form bars of widths greater than either bar. In order for the hammers to be individually operable, some spacing must exist between them which tends to cause the combined bar width to have some gap potential. U.S. Pat. No. 4,396,303 shows printing multiple width bar imprints by combining single width bars printed from single width print bars closely adjacent each other. This printer requires moving of the paper to get the close spacing of the bar imprints. Other examples of impact printing devices for printing bar code patterns with bar imprints of different widths are shown in U.S. Pat. Nos. 4,027,586 and 4,762,063 and the article of R. L. Gilbert in the IBM Technical Disclosure Bulletin of October 1974, Vol. 17, No. 5 pp. 1321-1322. These show type carriers with type bars of different widths which are impacted by print hammers. None of these items deal with the problem of printing multiple width bar patterns with single energy hammers.